Dimethyl ether (DME) has recently been identified as a potential fuel of the future. It has several similar properties to propane and liquefied petroleum gas (LPG), and can be used in blends with LPG up to 20% for combustion applications, such as heating and cooking, without modification of the end-use equipment. DME can also be used in blends or by itself in diesel engines after modifications with fuel injectors. In both cases, DME burns cleaner than the traditional fuel, with no particulate or sulfur emissions and very low CO and NOx emissions.
Traditionally, DME is produced from natural gas. Natural gas is reformed through reforming, either auto-thermal reforming or partial oxidation, to generate synthetic gas (syngas). The syngas is cleaned to remove impurities. The syngas is sent to two-step reactor system to convert the syngas to methanol and subsequently to DME. The DME is then purified for the desired grade. The production price of DME from this route will be heavily influenced by the present cost of natural gas. This has been a traditional limitation in the wider use of DME as a fuel product.
More interest in renewable sources has led to generating DME from renewable or inexpensive sources, including coal, coal-bed methane, and stranded natural gas. These streams can also include waste streams such as wood chips, black liquor, municipal solid waste (MSW), refuge-derived waste (RDF), plastics, paper, seed oil, and agricultural waste. When using solid waste feedstock like MSW, the solid material is converted to syngas through pyrolysis and/or gasification. The syngas is then cleaned and converted to DME as with the natural gas process to DME. The production cost using waste streams will be much lower than if using natural gas, and is seen as an advantageous route towards inexpensive DME. The availability of such waste streams may be limited in both amount and composition, and may not be sufficient to meet the increased demand for DME as a fuel.
A key issue with using waste streams, particularly MSW, to produce DME or other liquid fuels is that it will not have consistent composition. MSW has long-term, seasonal, and short-term variations in its composition. This in turn affects the quality of the syngas produced, specifically the ratio of hydrogen to carbon monoxide. For optimal DME production, this ratio is in a range between 1 to 2. DME production technology can tolerate small variation both high and low in those ratios. Wider variances can lead to inefficient and potentially unsafe operation. The variation of hydrogen-to-carbon monoxide ratio generated from MSW may be too large without additional processes and process controls to maintain safe and efficient DME production.